That's terrible! The ONLY way people should train dogs to be agressive is if they are Police Dogs. But that's just awful. My dad told me dogs just "know", when to be agressve and/or bite. Dogs should only bite of attack if someone is truly trying to hurt their master, or family, and if someone is trying to rob thier family's house. But, it's normal for a dog to bark or growl when someone is knocking at the door! Sadie only barks when someone is at the door, or when she hears a strange sound. People shouldn't do that.